A power transistor has been used for driving a fuel pump, an ignition coil drive apparatus and an injector drive apparatus of an internal combustion engine. Since such a transistor will likely be damaged by the generated heat of the transistor when the current conducting time of the transistor is too long, some counterplan for preventing the destruction of the transistor must be taken. For instance, such a counterplan is disclosed in FIG. 1 of Japanese Laid-Open No. 52-67425 entitled "Non-contactor type ignition apparatus" and published on Jun. 3, 1977. In this prior publication, the disclosed apparatus operates to detect the temperature rise of the power transistor during normal operation using a thermal sensitive element, such as a thermistor, which is located near the power transistor, and to control the current conducting time by varying the bias level of the power transistor based on the output of the thermal sensitive element, so that the power transistor is maintained below the thermal temperature limit thereof.
According to the prior art mentioned above, the current conducting time to the power transistor is controlled by the output signal of the thermistor which is located near the power transistor. When a large transient current flows during an engine stop condition and the temperature of the engine is high, the power transistor is heated abruptly. However, the temperature increase at that time can not be detected by the thermistor, since it is so abrupt, so that the temperature of the power transistor exceeds its temperature limit and the power transistor is damaged.